The many application of LEDs are very diverse but fall into three major categories: Visual signal application where the light goes more or less directly from the LED to the human eye, to convey a message or meaning. Illumination where LED light is reflected from object to give visual response of these objects. Finally LEDs are also used to generate light for measuring and interacting with processes that do not involve the human visual system.

Indicators and signs

• Status indicators on a variety of equipment
• LED displays used as stadium television displays, electronic billboards and dynamic decorative displays.
• Traffic lights and signals
• Exit signs
• Thin, lightweight message displays at airports and railway stations, and as destination displays for trains, buses, trams, and ferries.
• Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.
• LEDs of all colors, including yellowish white to simulate incandescent lamps, are used for model railroading applications
• In dot matrix arrangements for displaying messages.
• Because of their long life and fast switching times, LEDs have been used for automotive high-mounted brake lights and truck and bus brake lights and turn signals for some time, but many high-end vehicles are now starting to use LEDs for their entire rear light clusters. Besides the gain in reliability, this has styling advantages because LEDs are capable of forming much thinner lights than incandescent lamps with parabolic reflectors. The significant improvement in the time taken to light up (perhaps 0.5s faster than an incandescent bulb) improves safety by giving drivers more time to react. It has been reported that at normal highway speeds this equals one car length increased reaction time for the car behind. White LED headlamps are beginning to make an appearance.
• As a medium quality voltage reference in electronic circuits. The forward voltage drop (e.g., about 1.7 V for a normal red LED) can be used instead of a Zener diode in low-voltage regulators. Although LED forward voltage is much more current-dependent than a good Zener, Zener diodes are not available below voltages of about 3 V.
• Glowlights, as a more expensive but longer lasting and reusable alternative to glowsticks.
• Lumalive, a photonic textile
• Emergency vehicle lighting
• LED-based Christmas lights available in different colors and with low energy consumption.
• LED-modules provide LEDs in a more usable form to people with less knowledge of electronics and soldering: the actual LEDs are contained within in protective and mountable casing, and a lead enables connection to power supply, typically 12 volts. LED modules are available in a wide range of shapes, sizes and colors.

Lighting

• Replacement light bulbs
• Flashlights with low energy usage and high durability
• Lanterns
• Street lights
• Large-scale video displays
• Architectural lighting
• Light source for machine vision systems, requiring bright, focused, homogeneous and possibly strobed illumination.
• Automotive lighting on cars, motorcycles
• bicycle lights
• Backlighting for LCD televisions and lightweight laptop displays. Using RGB LEDs increase the color gamut by as much as 45%.
• Light source for DLP projectors
• Stage lights using banks of RGB LEDs to easily change color and decrease heating from traditional stage lighting.
• Medical lighting where IR-radiation and high temperatures are unwanted.
• Strobe lights or camera flashes that operate at a safe, low voltage, as opposed to the 250+ volts commonly found in xenon flashlamp-based lighting. This is particularly applicable to cameras on mobile phones, where space is at a premium and bulky voltage-increasing circuitry is undesirable.
• Invisible infrared illumination for night vision, such as many security cameras.
• A ring of LEDs around a video camera, aimed forward into a retroreflective background, will allow for chroma keying in video productions
Non-visual applications
• Grow lights using LEDs to increase photosynthesis in plants
• Remote controls, such as for TVs and VCRs, often use infrared LEDs.
• Movement sensors, for example in optical computer mice. The Nintendo Wii's sensor bar uses infrared LEDs.
• As light sensors
• In optical fiber and Free Space Optics communications.
• In pulse oximeters for measuring oxygen saturation
• LED phototherapy for acne using blue or red LEDs has been proven to significantly reduce acne over a three-month period.
• Some flatbed scanners use arrays of RGB LEDs rather than the typical cold-cathode fluorescent lamp as the light source. Having independent control of three illuminated colors allows the scanner to calibrate itself for more accurate color balance, and there is no need for warm-up. Furthermore, its sensors only need be monochromatic, since at any one point in time the page being scanned is only lit by a single color of light.
• As UV curing devices for some ink and coating applications as well as LED printers.
• Sterilization of water and other substances using UV light.
• Touch sensing: Since LEDs can also be used as photodiodes, they can be used for both photo emission and detection. This could be used in for example a touch-sensing screen that register reflected light from a finger or stylus.
• Opto-isolators use an LED combined with a photodiode or phototransistor to provide a signal path with electrical isolation between two circuits. This is especially useful in medical equipment where the signals from a low voltage sensor circuit (usually battery powered) in contact with a living organism must be electrically isolated from any possible electrical failure in a recording or monitoring device operating at potentially dangerous voltages. An optoisolator also allows information to be transferred between circuits not sharing a common ground potential.
Light sources for machine vision systems
Machine vision systems often require bright and homogeneous illumination, so features of interest are easier to process. LEDs are often used to this purpose, and this field of application is likely to remain one of the major application areas until price drops low enough to make signaling and illumination applications more widespread. Barcode scanners are the most common example of machine vision, and many inexpensive ones used red LEDs instead of lasers.
LEDs constitute a nearly ideal light source for machine vision systems for several main reasons:
• Size of illuminated field is usually comparatively small and Vision systems or smart camera are quite expensive, so cost of LEDs is usually a minor concern, compared to signaling applications.
• LED elements tend to be small and can be placed with high density over flat or even shaped substrates (PCBs etc) so that bright and homogeneous sources can be designed which direct light from tightly controlled directions on inspected parts.
• LEDs often have or can be used with small, inexpensive lenses and diffusers, helping to achieve high light densities and very good lighting control and homogeneity.
• LEDs can be easily strobed (in the microsecond range and below) and synchronized; their power also has reached high enough levels that sufficiently high intensity can be obtained, allowing well lit images even with very short light pulses: this is often used in order to obtain crisp and sharp “still” images of quickly-moving parts.
• LEDs come in several different colors and wavelengths, easily allowing to use the best color for each application, where different color may provide better visibility of features of interest. Having a precisely known spectrum allows tightly matched filters to be used to separate informative bandwidth or to reduce disturbing effect of ambient light.
• LEDs usually operate at comparatively low working temperatures, simplifying heat management and dissipation, therefore allowing plastic lenses, filters and diffusers to be used. Waterproof units can also easily be designed, allowing for use in harsh or wet environments (food, beverage, oil industries).
• LED sources can be shaped in several main configurations (spot lights for reflective illumination; ring lights for coaxial illumination; back lights for contour illumination; linear assemblies; flat, large format panels; dome sources for diffused, omnidirectional illumination).
• Very compact designs are possible, allowing for small LED illuminators to be integrated within smart cameras and vision sensors.